This invention relates to a process for separating bitumen from bitumen froth. More particularly, it relates to a process for separating bitumen by heating the bitumen froth at an elevated temperature and pressure and gravity separating the water and solid components from the bitumen froth.
The reserves of liquid hydrocarbons in bitumen deposits are very substantial and form a large portion of the world's known energy reserves. These deposits are relatively expensive to develop compared with conventional petroleum crude oils. The heavy oils are extracted from the deposits either by mining methods or in-situ steam injection. The mined ore is subsequently treated with steam, hot water and caustic in a hot water extraction process carried out at approximately 80.degree. C. to liberate the bitumen from the sand to form a froth. This froth contains a significant portion of water and solids which must be substantially reduced prior to an upgrading step. Heavy oil produced by in-situ methods also contain significant quantities of water and solids which must be treated prior to upgrading. The upgrading processes convert the heavy oil to lighter fractions which can be further processed into naphtha, gasoline, jet fuel and numerous other petroleum products.
These heavy oils and bitumen froths are deaerated to remove entrained air and treated to remove significant water and solids. The most common method of purification is to dilute the produced froth or heavy oil with naphtha to reduce the viscosity. With bitumen froth, the naphtha is added in approximately a 1:1 ratio on a volume basis. The diluted bitumen may then be subjected to centrifugation in two stages. In the first stage, coarse solids are removed using scroll type machines. The product from this step is then processed in disc centrifuges which remove a significant portion of the water and solids. The naphtha diluent is recovered from this bitumen by flash distillation and recycled to the froth treatment step to be reused. In the case of insitu produced heavy oils, a similar diluent is added to reduce the heavy oil viscosity for the purposes of separation of solids and water and subsequent pipeline transport. The removed solids and water are disposed of in a tailings pond or other containment area. The diluent may either be recovered for reuse, or it may be left in the recovered heavy oil to be used in pipeline transport of the product.
The bitumen or heavy oils are upgraded or refined in processes such as fluid coking, LC-Fining.TM., residuum hydrotreating or solvent deasphalting. It is desirable to eliminate essentially all the water and to reduce the solids to less than 1 per cent by weight before proceeding with any of these processes. The conventional specification for pipeline feed of oil is a maximum of 0.5 per cent bottom sediment and water (BS&W). With bitumen produced from froths by centrifugation following conventional mining and extraction processing, the solids content of the bitumen is seldom less than 1 per cent. From in-situ production, the specification of 0.5 per cent BS&W may be achieved
Various methods have been used to remove water and solids from such froths. Given et al (U.S. Pat. No. 3,338,814) describe a process whereby froths produced by hot water extraction of bitumen are dehydrated by heating to temperatures from 225.degree. F. to 550.degree. F. (preferably 350.degree. F. to 450.degree. F.). The dehydrated bitumen, containing 5% to 25% solids is then subjected to cycloning or filtration to remove solids. In a variation to the basic process, a light hydrocarbon can be added to the dry bitumen to improve the filtration step. The hydrocarbon can be recovered by distillation and recycled. This is essentially a two-stage process that requires a considerable amount of energy in order to obtain a satisfactory degree of extraction.
Another attempt to remove water and solids from bitumen froths was disclosed by Leto et al (U.S. Pat. No. 4,648,964). In this process, bitumen froths are heated to temperatures above 300.degree. C. at pressures above 1000 psig to heat/pressure treat the froth prior to separation of a hydrocarbon layer from water and solids at atmospheric pressure in a second stage. The second stage of the process requires pressure reduction and cooling of the material to a temperature about 80.degree. C. Naphtha, in a weight ratio of naptha to treated stream in the range of 0.5-1.0:1.0, is then added to the bitumen and the mixture is introduced into a gravity separation vessel at atmospheric pressure. The hydrocarbon is withdrawn from the top of the vessel and a solids and water fraction is removed from the bottom. The bottoms are transferred to a second settling vessel where clarified water is withdrawn from the overflow and solids are removed from the underflow for disposal. Since this process requires extremely high temperatures and pressures and a relatively intricate apparatus for controlling the changing temperatures and pressures from a high pressure separation in a first stage to atmospheric pressure separation in a second stage of the two-stage process, the extraction costs are relatively high.
Baillie (Canadian patents 952,837, 952,838, 952,839 and 952,840) discloses embodiments of a method for upgrading bitumen froth in which diluted bitumen froth recovered from a scroll centrifuge is heated to a temperature in the range of 300.degree.-1000.degree. F. and transferred to an autoclave settling zone for settling at a pressure in the range of 0-1000 psi. The tailings are cooled and passed to a disc centrifuge for secondary recovery at ambient pressure. This process requires the use of expensive centrifuges which are costly to operate and maintain and which are prone to shut-down due to wear because of the erosive nature of the material treated. In addition, the method requires the use of higher boiling liquid hydrocarbon diluents boiling in the range of 350.degree.-750.degree. F. and necessitates the steps of pressure reduction and cooling for the secondary stage disc centrifugation at atmospheric pressure.
A process developed by Shelfantook et al (U.S. Pat. No. 4,859,317) as an alternative to conventional dilution centrifuging circuits for purifying bitumen froths proposes three states of inclined plate settlers to remove water and solids from bitumen froths. This process is carried out at approximately 80.degree. C. using naphtha as diluent in a 1:1 volume ratio based on the oil content in the froth. The lower temperature operation however results in a diluted bitumen product which contains a significant quantity of solids. The residual solids are at substantially higher levels than the specification required for pipelining and for some refining processes.
It is a principal object of the present invention to provide an improved process for effectively separating the bitumen component from the water and solids components of a bitumen froth by treating the froth at a relatively moderately elevated temperature and pressure, and gravity separating the said components while maintaining said temperature and pressure.
It is another object of the present invention to provide an improved process for separating the bitumen component from the water and solids of a bitumen froth using a substantially constant elevated temperature and pressure during the separation stages for enhanced recovery of bitumen with the use of simple and relatively inexpensive gravity separation equipment.